Air-gap optical structures are used in optical systems. In an example, two prisms separated by an air gap are used in some designs of reflective digital micromirror device (DMD) projectors. The air-gap optical structure provides for total internal reflection at an air-gap face for some incident angles of light, but transmission through the air-gap face for other incident angles of light. In the projector, an incident beam is reflected from the air-gap face to the reflective DMD display under conditions of total internal reflection, reflected from the DMD display according to the image thereon and back toward the air-gap face, and passed through the air-gap face under transmission conditions for viewing.
The air-gap optical structure is formed by providing two transmissive optical elements (such as prisms or lenses) of the proper shapes, and placing them together in a facing relationship with the air gap defined between the adjacent faces. A number of techniques have been used to form the air gap. For example, in various instances metal shims, deposited layers, and other types of spacers have been positioned between the adjacent faces of the transmissive optical elements to define the air gap.
One of the problems with such air-gap optical structures is stray light produced by reflections within the air-gap optical structure. Some of the stray light may find its way through the projector, reducing the signal-to-noise ratio in the projected image. A number of techniques are candidates for reducing the stray light, such as the use of very careful design and manufacturing techniques that account for stray light and direct the stray elsewhere than into the projected image, and the use of light baffles of various types. In each case, a significant system redesign is required to utilize these techniques for the reduction of stray light, and the redesigns place constraints on the structure and manufacturing process for the air-gap optical structure.
There is a need for an understanding of the source of the stray light, and for an associated less-restrictive, more-general approach for reducing stray light within the air-gap optical structure. The present invention fulfills this need, and further provides related advantages.